An example of a digital printing system as set out above is described in detail in WO 2013/132418 which discloses use of a water-based ink and an ITM having a hydrophobic outer surface.
In indirect printing systems, it is common to wrap the ITM around a support cylinder or drum and such mounting ensures that, at the image forming station, the distance of the ITM from the print bars does not vary. Where, however, the ITM is a driven flexible endless belt passing over drive rollers and tensioning rollers, it is useful to take steps to ensure that the ITM does not flap up and down, or is otherwise displaced, as it passes through the image forming station and that its distance from the print bars remains fixed.
In WO 2013/132418, the ITM is supported in the image forming station on a flat table and it is proposed to use negative air pressure and lateral belt tensioning to maintain the ITM in contact with its support surface. In some systems, employing such construction may create a high level of drag on the ITM as it passes through the image forming station.
In WO 2013/132418, it is also taught that to assist in guiding the belt smoothly, friction may be reduced by passing the belt over rollers adjacent each print bar instead of sliding the belt over stationary guide plates. The rollers need not be precisely aligned with their respective print bars. They may be located slightly (e.g. few millimeters) downstream of the print head jetting location. Frictional forces are used to maintain the belt taut and substantially parallel to print bars. To achieve this, the underside of the belt has high frictional properties and the lateral tension is applied by the guide channels sufficiently to maintain the belt flat and in contact with rollers as it passes beneath the print bars.
Some systems rely on lateral tension to maintain the belt in frictional engagement with the rollers to prevent the belt from lifting off the rollers at any point across. Nevertheless, in some systems, this may increase (even severely) the drag on the belt and wear of the guide channels.